As a system which wirelessly transmits power (a wireless power transmission system), a system using an electromagnetic induction phenomenon is being explored. In addition to the system using the electromagnetic induction phenomenon, a system using a resonance phenomenon of magnetic field energy (magnetic field resonance) has also been proposed as a wireless power transmission system. The system using the magnetic field resonance has characteristics that no trade-off relation occurs between transmission distance and transmission efficiency, as compared with the wireless power transmission system using the electromagnetic induction phenomenon. Therefore, the system using the resonance of the magnetic field energy is expected to be a promising technology for charging an electric apparatus such as a cellular phone and an EV (Electric Vehicle).
In the wireless power transmission system using the magnetic field resonance, it is important to have a technology for compensating for a change in power transmission characteristics which results from changes in the relative positional relation between a power transmitting antenna and a power receiving antenna, or the condition of a medium existing between antennas (hereinafter, also called a transmission medium). This is because, when the relative positional relation between the power transmitting and receiving antennas or the condition of the medium is changed, the amount of magnetic flux passing through the power receiving antenna is changed, resulting in a change in a frequency and a load at which maximum power transmission efficiency is obtained (an optimal frequency and an optimal load, respectively).
FIG. 10 is a conceptual diagram illustrating a relation between power transmission efficiency and frequency in a resonance system. FIG. 10 illustrates the relation between power transmission efficiency and frequency in the cases where the distance between the power transmitting antenna and the power receiving antenna is a DISTANCE a (a solid line) and a DISTANCE b (a broken line). That is, according to the distance between the power transmitting and receiving antennas, the relation between power transmission efficiency and frequency changes, resulting in a shift of an optimal frequency at which power transmission is maximum. In addition, it has been known that such a shift of the optimal frequency is caused by a change in a resonance condition, that is, a change in a coupling condition of magnetic flux.
A technology for compensating for such a change in the resonance condition is particularly important as a technology for efficiently charging a movable body such as an EV. For example, when an EV is parked close to a source installed on the ground, the parking position of the EV varies depending on the driver. Therefore, the relative positional relation (the distance and angle) between the power transmitting antenna mounted on the source installed on the ground and the power transmitting and receiving antennas mounted on the EV changes every time the EV is charged. Accordingly, it is important to have a technology for compensating for a change in the resonance condition (a change in the coupling condition of the magnetic flux).
In order to compensate for the change in the resonance condition, a circuit system (a control device) has been considered which adjusts circuit constants (capacities) of a power transmitting antenna and a power receiving antenna so as to optimally adjust frequencies of the antennas and loads. Moreover, a system has been known which, in order to enhance the accuracy of compensation and responsiveness, calculates a control direction for an optimal frequency, that is, a system that determines, by calculating differential values of power transmission characteristics with respect to frequency, whether the control should be performed to increase or to decrease the frequency so as to arrive at an optimal frequency. An example of such a control device, for example, is disclosed in PTL1.
PTL1 discloses a system, a method, and an apparatus for adjusting a wireless power transmitter. The wireless power transmitter includes as an example a transmission circuit having a transmission coil and wirelessly supplies power to a load by the transmission circuit. Herein, the transmission coil resonates at a predetermined resonant frequency. Furthermore, the transmission circuit has a circuit constant (reactance). The wireless power transmitter includes a detection circuit that detects a change in a resonant frequency while the power is being supplied to the load and can adjust the reactance on the basis of a change in the resonant frequency.